GLAS/ICESat L2 Sea Ice Altimetry Data (HDF5), Version 34

GLAH06 is used in conjunction with GLAH05 to create the Level-2 altimetry products. Level-2 altimetry data provide surface elevations for ice sheets (GLAH12), sea ice (GLAH13), land (GLAH14), and oceans (GLAH15). Data also include the laser footprint geolocation and reflectance, as well as geodetic, instrument, and atmospheric corrections for range measurements. The Level-2 elevation products, are regional products archived at 14 orbits per granule, starting and stopping at the same demarcation (± 50° latitude) as GLAH05 and GLAH06. Each regional product is processed with algorithms specific to that surface type. Surface type masks define which data are written to each of the products. If any data within a given record fall within a specific mask, the entire record is written to the product. Masks can overlap: for example, non-land data in the sea ice region may be written to the sea ice and ocean products. This means that an algorithm may write the same data to more than one Level-2 product. In this case, different algorithms calculate the elevations in their respective products. The surface type masks are versioned and archived at NSIDC, so users can tell which data to expect in each product. Each data granule has an associated browse product.

Version Summary:

Release 34 incorporates fixes for several data issues that were determined to exist in the GLAS Release 33 data products.

Detailed Data Description

NOTE: References in this documentation to GLAS binary product names GLA01 to GLA15 refer to original GLAS binary data, and are retained for informational and provenance purposes. Access to GLAS binary data was removed 01 August, 2017. All GLAS data are available in HDF5 format, products GLAH01 to GLAH15.

Note: GLAH01 Release 33 did not require Surface Elevation reprocessing, so was not reprocessed into Release 34, and remains as Release 33.2. The 33.2 minor version revises the format of the d_4nsBgMean parameter from counts to volts. GLAH01 contains all altimetry information transmitted from the spacecraft, including the long and short waveforms. The number of received samples is either 200 or 544. These change at the frame boundary and are nominally set by the onboard surface-type mask. In normal operations, GLAS receives 200 samples over sea ice and ocean, and 544 samples over ice sheet and land. The transmit pulse, received echo samples, and associated digitizer addresses are transferred from Level-0 telemetry without calibration or unit changes. This is the only product that contains the altimeter transmitted and received waveforms, which may be required by altimetric scientists investigating the instrument health. This product is not intended for use by the general science community.

GLAH05 is an intermediate product that contains important information calculated from the waveform. Level-1B waveform parameterization data include output parameters from the waveform characterization procedure and other parameters required to calculate surface slope and relief characteristics. The binary GLA05 was used for creating GLA06 and Level-2 elevation products. The higher products contain scientific parameters derived from algorithms that specifically used GLA05 as input. GLAH05 contains parameterizations of both the transmitted and received pulses and other characteristics from which elevation and footprint-scale roughness and slope are calculated. The received pulse characterization uses two implementations of the retracking algorithms: one tuned for ice sheets, called the standard parameterization, used to calculate surface elevation for ice sheets, oceans, and sea ice; and another for land (the alternative parameterization).

GLAH06 is a product that is analogous to the geodetic data records distributed for radar altimetry missions. It contains elevations previously corrected for tides, atmospheric delays, and surface characteristics within the footprint. Elevation is calculated using the ice sheet parameterization. Additional information allows the user to calculate an elevation based on land, sea ice, or ocean algorithms. Level-1B elevation data include surface elevation, surface roughness assuming no slope, surface slope assuming no roughness, and geodetic and atmospheric corrections for range measurements. Both Level-1B products are geolocated to the center of the laser footprint.

GLAH12 to GLAH15. GLAH06 is used in conjunction with GLAH05 to create the Level-2 altimetry products. Level-2 altimetry data provide surface elevations for ice sheets (GLAH12), sea ice (GLAH13), land (GLAH14), and oceans (GLAH15). Data also include the laser footprint geolocation and reflectance, as well as geodetic, instrument, and atmospheric corrections for range measurements. The Level-2 elevation products, are regional products archived at 14 orbits per granule, starting and stopping at the same demarcation (± 50° latitude) as GLAH05 and GLAH06.

Surface Type Mask

Each regional product is processed with algorithms specific to that surface type. Surface type masks define which data are written to each of the products. If any data within a given record fall within a specific mask, the entire record is written to the product. Masks can overlap: for example, non-land data in the sea ice region may be written to the sea ice and ocean products. This means that an algorithm may write the same data to more than one Level-2 product. In this case, different algorithms calculate the elevations in their respective products. The surface type masks are versioned and archived at NSIDC, so users can tell which data to expect in each product.

Format

GLAS HDF5 products are in Hierarchical Data Format 5 format, also known as netCDF-4/HDF5. The intent with the HDF5 format data is to make the Release 34 data available in the same format as several future NASA missions, including ICESat 2 and Soil Moisture Active Passive (SMAP), and to make ICESat 1 Release 34 data more accessible to a broader user community.

Header

The header description information found in the binary files is embedded in the HDF5 files.

Invalid values

Not all data from GLAS are suitable for science processing. Many parameters have invalid values or, in CF terms, "_FillValues". An invalid value means that the measurement is not valid for that element of data.

Surface-type mask

Surface-type mask data (available via HTTPS) are stored as an 10800 by 5400 byte array, where each byte represents the surface type for one 30 second grid cell on the earth. Each 10800-byte record represents a 360° latitude band. The first record starts at the North Pole, and the records follow at 30 second intervals to the South Pole.

Valid pixel values range from 1 to 15. The surface types are bit-coded, and any combination of surface type is allowed. The bit coding is:

Each data granule has associated browse products that users can quickly view to determine the general quality of the data in the granule. Browse products consist of image plots of key parameters and statistics.

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File Naming Convention

The file naming convention is as follows:

GLAH05_634_2131_002_0084_4_01_0001.H5

GLAHxx_mmm_prkk_ccc_tttt_s_nn_ffff.h5

Where:

Table 2. File Naming Convention

Variable

Description

GLAH

Indicates GLAS HDF5 data product

xx

Product number (01, 05, 06, 12, 13, 14, or 15)

mmm

Release number for process that created the product = 634

p

Repeat ground-track phase (1 = 8-day, 2 = 91-day, 3 = transfer orbit)

r

Reference orbit number; this number starts at 1 and increments each time a new reference orbit ground track file is obtained.

kk

Instance number, incremented every time the satellite enters a different reference orbit.

ccc

Cycle of reference orbit for this phase; the cycle number restarts at 1 every time the instance number changes. The cycle number then increments within the instance (kk) every time Track 1 for that orbit is reached. Most instances begin in an arbitrary track (not 1) because of how the tracks are numbered.

tttt

Track within reference orbit; tracks are defined from a reference orbit. Each track begins and ends at the ascending equator crossing. Tracks are numbered such that Track 1 is the closest track to Greenwich Meridian from the east and then contiguous in time after that. For transfer orbits, for which we have no predefined reference orbit, Track 1 is the first track for which we have data for that instance (kk).

s

Segment of orbit

nn

Granule version number; the number of times this granule is created for a specific release

ffff

File type; numerical, assigned for multiple files as needed for data of same time period for a specific data product; a multifile granule

h5

HDF5 file

Algorithms that generate altimetry products are continually being improved, as limitations become apparent in early versions of data. As a new algorithm becomes available, a new release number (mmm) becomes available. Users are encouraged to work with the latest release. Release-34 is the newest and final release.

Note: Beginning with Release-28, a new convention is used for the release number (mmm) in file names.

Spatial searching is disabled for GLAH01 (V33.2). The orbit for GLAH01 is a predicted orbit and does not show any target-of-opportunity pointing. Spatial searching for GLAH01 is not enabled in because of a potentially large number of false negatives. If you order GLAH05 to GLAH15 data granules (which support spatial searching) and you require matching GLAH01 granules, please note the data times and/or file names from the GLAH05 to GLAH15 granules; use these times to perform your search for GLAH01 granules.

Spatial Resolution

GLAS is a profiling instrument that collects data only where the altimeter points, nominally along the ground track. At 40 pulses per second, the centers of 60 m spots illuminated by the laser on the earth's surface are separated in the along-track direction by 172 m from a 600 km altitude orbit.

Browse products contain quality information. These browse products are available along with corresponding data granules.

Browse images that accompany each data file contain the statistics listed below. Quality control for land and ice sheet products begins with calculating the percent of measurements for which no signal was found. For those measurements in which a signal was found, the following are tabulated (Brenner et al. 2000):

Percentage of measurements for which the fitting procedure is successful

Percentage of measurements that may be degraded from detector saturation

Each data granule has a set of histograms displaying statistical information about waveform characteristics. These are only provided for signals with successful fits:

Differences between the centroid of the received waveform and the centroid of the Gaussian fit to the last peak

The GLAS Science Team has developed a set of procedures for verifying and calibrating Level-1 science products to ensure that appropriate geophysical interpretations can be drawn from the data products and to reduce the level of geophysical uncertainty in the data. A formal calibration-validation (CV) plan is pending. More information will be available in the near future.

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Data Acquisition and Processing

The ICESat GLAS Release 34 HDF5 data are converted directly from the Release 34 binary data. Data files were not re-processed during conversion. However, parameters in the HDF5 files are re-ordered, re-named, and logically grouped to take advantage of the HDF5 file structure.

Error Sources

GLAS Release 34 Correction of Release 33 Data Issues

Correction to the ICESat Data Product Surface Elevation due to an Error in the Range Determination from Transmit-Pulse Reference-Point Selection (Centroid vs Gaussian)

It was determined that an important correction to the surface elevations on the ICESat products was not applied. The range from which the surface elevation should have been calculated is from the midpoint of the Gaussian peak on the transmit pulse to the midpoint of the Gaussian peak on the received pulse. However the location of the centroid of the transmit pulse was inadvertently used and the difference (defined as G–C) between the transmitted pulse centroid and Gaussian peak was never applied. The effect of this error of omission varies on a shot-to-shot basis and for the calibration passes over the Salar de Uyuni in Bolivia varied from ± 6 cm over the mission. Similar results were found when tabulating the track-averaged corrections (G–C) over Antarctica. Note that the (G–C) values are fairly constant within a campaign, so any elevation data adjustments made to data release 633 or older for "campaign-level" biases that were determined to an independent reference surface would eliminate the G–C error from the elevation data on an average basis.

The surface elevations on GLA06 (global elevation product), GLA12 (ice sheet), GLA13 (sea ice), and GLA15 (ocean) are affected. The GLA14 (land) elevations are computed using the centroid of the received pulse, as well as the centroid of the transmit pulse, and therefore the elevation calculation on GLA14 is correct as intended. However, GLA14 data users who use the range Increment for the up to six Gaussian fitted peaks may want to use the G–C correction also.

Dry Troposphere Correction Jitter

It was reported that at times the dry troposphere range correction contained several centimeters of jitter. The issue was traced to the threshold range being used to determine the dry troposphere correction in regions of rapidly changing shape of the return signal waveforms. This was normally in poor signal areas or very rough terrain. At times the threshold range relative to the standard fit would jump tens of meters and induce several centimeter changes in the dry troposphere correction. Changes were made to use the centroid range to determine the troposphere correction. The centroid closely follows the standard fit range.

During the investigation it was determined that the GSAS code was using only one of the 6–hour met files for the pressure inputs. Code was modified to properly interpolate between the two 6–hour met files.

The GLAS product high resolution DEM was determined to have issues in the Southern Latitude when the source was SRTM data. It was reported to have wrong values that showed as banding.

There was an interpolation error in the southern latitudes during the creation of the SRTM track files (ANC51). The code error caused the DEM values to be correct at the mid point latitude between the interpolation points, with increasing errors on either side. This resulted in the banding error seen when comparing GLAS SRTM DEM values with the source SRTM DEM. The higher the DEM gradient the larger the error so they were significant over Australia. The code was fixed and track files were re–created.

The order of preference for which values are used when the SRTM and CDED overlap was changed. Starting with this release the SRTM is always used in the overlap region.

Some parameters on GLA14 were invalid when i_elev was valid.

It was determined that if any standard fit for a one second was invalid, checks were not made to determine if GLA14 had valid elevations during that second. Changes were made to provide the parameters based on a valid alternate fit if the standard fit was invalid for a shot.

It was determined that the atmosphere confidence flag from GLA09 was not being placed on GLA12, 13, 14, and 15. Changes were made to correctly put the confidence flags on the products.

Occasional mismatch of GLA09 atmosphere characteristic flag value and the value reported on GLA06 and 14.

On occasion the atmosphere characteristic flag was zero on GLA06 and 14 when a non-zero value was on GLA09. It was determined that the time tolerance check for the one second GLA06 time to the GLA09 time tag did not allow for sufficient jitter between the time tags. This caused a GLA09 record not to be selected as the source for the values. Several other parameters were also found to be invalid.

Document Information

DOCUMENT CREATION DATE

DOCUMENT REVISION DATE

10 June 2014

17 January 2013

08 October 2014

23 October 2014

No technical references available for this data set.

FAQ

Are the altimetry data adjusted to the Geoid, or only to the Ellipsoid?

The elevation data are relative to the ellipsoid.
The data also have a parameter indicating the geoid height, which is the height of the EGM2008 geoid above the TOPEX/Poseidon ellipsoid for the first and last shot in the record:
GLAH06
Data_40HZ/Geophysical/d_gdHt
GLAH12
Data_40HZ/... read more

What is the accuracy of GLAS elevation measurements?

Surface roughness greatly affects the elevation measurement accuracy. Over most of the ice sheets, the accuracy of each elevation measurement is 15 cm. Average ice elevation changes less than 1 cm will be detected by averaging the elevation differences observed at many points in selected regions of... read more

How is the surface elevation at each laser footprint determined?

In short, the elevation of the surface at each laser footprint is the height of the spacecraft minus the measured distance to the surface.
A standard parameterization is used to calculate surface elevation for ice sheets, oceans, and sea ice, using the elevation of the maximum peak and no more... read more

In the GLAS altimetry products, is there an indicator to the presence of clouds?

Yes, it is an element of GLAH06, GLAH12, GLAH13, GLAH14, and GLAH15.
The Multiple Scattering Warning Flag, cld1_mswf_flg, is based on the total column optical depth (aerosol plus cloud) calculated in GLAH11 using 532nm. It is intended as a way to quickly obtain information about the potential... read more

What is the basic difference between a standard and alternate waveform fit and when would you normally choose one over the other?

The standard waveform fit is optimized for "ice sheet-like" returns. The majority of these are single-peaked, narrow waveforms. The standard fit allows for only two peaks. Alternate waveform fitting is meant to capture up to six peaks. More peaks are necessary for more complex waveforms over land,... read more

How does GLAS geolocate data and provide off-nadir pointing?

Analysis of altimetric data acquired by the GLAS instrument requires accurate determination of the laser spot location on the Earth's surface (ice, land, water, clouds) or geolocation of the laser spot. The spot location with respect to the Earth's center of mass (geocenter) is determined by both... read more

What data subsetting and reformatting services are available for ICESat/GLAS data?

The following table describes the data subsetting and reformatting services that are currently available for ICESat/GLAS HDF5 data via the NASA Earthdata Search tool.
Short name
Title
Subsetting
Reformatting... read more

How To

How do I programmatically request data services such as subsetting, reformatting, and reprojection using an API?

The subsetting, reformatting, and reprojection services provided by NSIDC through NASA Earthdata Search can also be accessed programmatically as a synchronous REST interface. This programmatic access is provided via an HTTPS URL containing a series of... read more

How to read GLAS HDF5 data into MATLAB, IDL, Python, or NCL

The HDF Group has example code for access and visualization of GLAS HDF5 data in MATLAB, IDL, Python, and NCL.

"Freeboard" is a measure of the height of sea ice above the ocean surface. The geoid height field ("d_gdHt") cannot be effectively used as a proxy for sea level in order to estimate freeboard, because current models of the Earth's geoid are too coarse to account for the local variations in geoid... read more

How do I order subsetted ICESat/GLAS HDF5 data in Earthdata Search?

Last updated: 5 July 2017
NASA Earthdata Search is a map-based search and order tool, allowing you to search for ICESat/GLAS data based on spatial and temporal constraints. Additionally, you can subset files based on spatial bounding box,... read more